Scanning electron microscopes are used in many applications that require inspection of very small structures of an object in great detail. Some of these applications include defect review and inspection of specimens such as very large scale integrated (VLSI) circuits, or wafers, or other articles, critical dimensioning of features in these specimens and also design and process verification of the specimens. Scanning electron microscopes are considered superior to optical microscopes for viewing features in sub-micron dimensions, currently about 0.2 μm (1 μm=10−6 meters) or less, due to the short wavelength that helps the scanning electron microscope to generate a small spot size. Scanning electrons microscopes typically employ an objective lens system for focusing the electron beam onto the specimen under examination.
A scanning electron microscope typically includes a primary electron beam source, an accelerating anode, an objective lens for focusing the beam onto the specimen, a plurality of deflection units that enable the positioning and scanning movement of the primary beam over the specimen, and a detection system for capturing secondary electrons (SE) or backscattered electrons (BSE) from the specimen to produce an image of the specimen. In some cases a condenser lens system is used to provide a focused beam for the objective lens system. As known to those skilled in the pertinent art, the electron beam source generates a supply of electrons for the primary beam. The condenser lens system, if used, forms an image of the primary electron beam source for the objective lens and the objective lens focuses the condenser lens image onto the specimen. The deflection system moves the focused beam over a portion of the specimen in a scanning motion and secondary and backscattered electrons are released from the specimen material. These electrons are detected, amplified and the resulting signal used to modulate the beam of an imaging system operating synchronously with the scanning electron beam. The result is an image of the scanned area based on the electrons emitted or scattered from the specimen.
Prior art scanning electron microscopes have several drawbacks. First, in order to obtain high resolution for the prior art objective lens, a relatively high beam energy, say 15 KeV or more, primary electron beam source is required. However, applying such a higher energy electron beam directly to the specimen is undesirable because it can cause damage to the specimen, which consequently leads to a reliability problem in the engineering, manufacturing and production of integrated circuits. Using a low energy primary electron beam source avoids the reliability problem but limits the spatial resolution due to chromatic aberration of the objective lens and by electron-electron interaction within the beam. Chromatic aberration of the objective lens arises from electrons of different velocity experiencing different focal points for the same lens. This effect creates a disk of confusion at the image plane on the specimen and limits resolution of the system. Both electron-electron interaction (space-charge effect) and chromatic aberration are reduced by using a higher energy electron beam. Second, it is difficult to achieve high secondary and backscattered electron capture efficiency with a low energy primary electron beam source. If a multi-channel plate is used, there is a severe reliability problem due to contaminants collecting in the holes (channels) of the plate.
As stated above, spatial resolution of current low voltage scanning electron microscopes is essentially limited by the chromatic aberration of the objective lens and by the electron-electron interaction, or Boersch effect. One way to improve the spatial resolution is to reduce the electron-electron interaction by using a high-energy primary bean which is subjected to a retarding field. Another way to improve spatial resolution is to use a snorkel (center pole) magnetic lens to reduce the loss in resolution due to chromatic and spherical aberration.
Conventional retarding field scanning electron microscopes which are equipped with a snorkel magnetic lens, a retarding electrostatic plate and pre-lens double deflection units are currently used for critical dimension measurement, defect review and defect inspection system in VLSI (very large scale integrated) wafer manufacturing. These conventional units still have several drawbacks.
First, it is difficult to generate a large deflection field with these units, where the deflection field is the reachable area in the plane of the specimen over which the primary beam can be moved by the deflection system. As a consequence, only a relatively small portion of the specimen can be examined at a time without repositioning the specimen. Second, because of the inherently small deflection field, the time required for inspecting, reviewing and measuring a specimen is substantially long. This results in increased engineering, testing, troubleshooting, and production costs, as well as an increase in integrated circuit turnaround time.
Thus, there is a need for an objective lens for an electron scanning microscope and method therefore that provides high primary beam current, lower energy electron beam on the specimen, relatively high resolution, a relative large scanning field, and a high signal capture efficiency.